The invention relates to lead titanate dielectrics in which at least 20 mol % of the titanium is substituted by magnesium and tungsten. The invention further relates to a method of manufacturing these dielectrics and to multilayer capacitors comprising the dielectric.
Dielectrics of the above type are of importance for the manufacture of capacitors, in particular multilayer capacitors. Monolithic ceramic multilayer capacitors can have very high capacities in a small volume and with great reliability. In manufacturing such capacitors the ceramic starting material is processed with a binder into thin ceramic foils. A metal paste is then provided on these ceramic foils to form the electrodes and subsequently the foils are stacked so that ceramic and metal layers alternate with each other.
Since the succession of layers of dielectric and metal lazers forming electrodes manufactured in this manner is sintered as one assembly, the electrode material and the sintering conditions must be chosen to be so that the metal layers neither melt nor oxidize.
It is known to manufacture multilayer capacitors in which the ceramic material is densely sintered in air at temperatures above 1300.degree. C. At these sintering temperatures only noble metals having a very high melting-point for example palladium or platinum, can be used as metals for the electrodes.
Since noble metals are very expensive and relatively scarce and moreover, since their price, bears no relation whatsoever to that of the ceramic material, also since as a result of the already reached high extent of mechanization in manufacturing multilayer capacitors, the cost of the materials for electrodes from noble metal is particularly important, various attempts have been made to manufacture ceramic materials with useful dielectric values but at drastically reduced sintering temperatures in order to be able to dispense with the expensive electrodes of palladium or platinum and instead thereof to use silver electrodes. A ceramic dielectric comprising PbTiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 is known from U.S. Pat. No. 4,063,341. This dielectric, which can be sintered at temperatures from 950.degree. to 1000.degree. C., has a dielectric constant .epsilon.&lt;1000 and exhibit values for the dielectric loss factor tan .delta. of 0.85 to 4.2% shows useful values. For the manufacture of multilayer capacitors from this ceramic material, teaches that the U.S. Pat. No. 4,063,341, silver should be used as a metal for the electrodes. It has now been found, however, that multilayer capacitors cannot be manufactured from this ceramic material in a reproducible quality while using silver electrodes since the sintering temperatures of this ceramic material is too close to the melting-point of the silver. However, because of its electrical properties, silver is excellently suitable as an electrode metal and, if possible, its use as an electrode should be maintained.